The present invention relates to a method for controlling the directionality of the sound receiving characteristic of a hearing aid comprising spaced apart first and second sound receiving microphone means, a signal processor for processing signals supplied by said microphone means and an output transducer for emission of sound signals in response to output signals from the signal processor, said method comprising the steps of changing over said sound receiving characteristic between an omnidirectional characteristic and a directional characteristic and, when operating the hearing aid with said directional characteristic, combining the signals supplied by said first and second microphone means into an overall combined signal, which is supplied to the signal processor, an adjustable time or phase delay being imposed on at least one signal.
Hearing aids having a directional sound receiving characteristic are useful to improve speech perception in noisy environments, where human speech may be received simultaneously from different directions, as is the case e.g. in the noise environment frequently referred to as cocktail party noise.
With a directional sound receiving characteristic, e.g. in the shape of a cardioid or super cardioid characteristic, the speech perception in a hearing aid is improved by reduced reception of sound coming from the back of the user, while maintaining the level of sound coming from the area in front of the user.
On the other hand, in environments with only a low noise level or no significant speech signals the hearing aid user will normal prefer an omnidirectional or spherical sound receiving characteristic offering the same perception of sound irrespective of the direction, from which it arrives.
As will be further explained in the following a prior art hearing aid of the kind defined above, offering the possibility of changing the sound receiving characteristic between an omnidirectional characteristic and a directional characteristic of varying shape has been disclosed in U.S. Pat. No. 5,757,933.
With this prior art hearing aid operating with an omnidirectional characteristic only the signal from the first microphone facing the area in front of the user is supplied to the signal processor. By manual operation of a switch a signal derived from the second microphone facing the rear of the user and subjected to inversion followed by adjustable phase delay and adjustable attenuation is combined via a summing node with the signal derived from the first microphone.
When the sound receiving characteristic in a hearing aid of this type is changed or changes from the omnidirectional to a directional shape, the arrival time of the sound changes during the transition. This change of phase or time delay may become confusing in a binaural hearing aid system using a pair of separate hearing aids operating with independent and automatic change of the sound receiving characteristic. When phase or arrival times change differently in the two hearing aids this will degrade or deteriorate the user""s ability to locate the various sound sources in the surrounding space and the advantage of a binaural hearing aid system will be degraded.
Furthermore, the phase and time relationship in a hearing aid degrades the quality of the sound perceived by the user. It may sound like the result of a Doppler-effect.
At the same time, in hearing aids of this type also the amplitude characteristic will change during transition between the omnidirectional and a directional characteristic, e.g. from a flat response to a response in which the amplitudes of higher frequencies will be increased. This increase may be in the area of 6 dB/octave. This results in the serious problem, that hearing aids of this type can not be perfectly fitted with an optimum transfer characteristic for both the omnidirectional and the directional characteristic.
On this background, it is the object of the present invention to provide a method of the kind defined, in which the deficiencies of the prior art hearing aid are remedied by effecting a smooth change-over between the omnidirectional characteristic and any directional characteristic substantially without changing the phase relationship or time delay and the amplitude characteristic of the signals. The change-over between the omnidirectional characteristic and a directional characteristic and vice versa may be controllable or even automatic.
According to the invention this object is achieved by a method of the kind defined, which is characterized in that said change-over of the sound receiving characteristic from the omnidirectional characteristic to the directional characteristic and vice versa is effected by controlled attenuation and time or phase delay of signals deriverd from both of the signals (Xfront, Xback) from the first and second microphone means before forming said overall combined signal (Y), using an adjustable attenuation control parameter (omni) and a delay (T), whereby said overall combined signal (Y) is determined by
xe2x80x83Y=Xfront*(1xe2x88x92omni*exe2x88x92jxcfx89T)+Xback*(omnixe2x88x92exe2x88x92jxcfx89T),
to change over the hearing aid between said omnidirectional characteristic and any desired form of said directional characteristic as a smooth change-over substantially without affecting phase relationship, time delay and amplitude characteristic of the hearing aid.
For carrying out the method as defined the invention further relates to a hearing aid with controllable directionality of its sound receiving characteristic, comprising spaced apart first and second sound receiving microphone means, a signal processor for processing signals supplied by said microphone means and an output transducer for emission of sound signals in response to output signals from the signal processor, and further comprising change-over control means for change over of the sound receiving characteristic between an omnidirectional characteristic and a directional characteristic and combining means for combination of the signal from the first and second microphone means to provide an overall combined signal supplied to the signal processor, when operating the hearing aid with said directional characteristic, and adjustable time or phase delay means being provided for producing a phase-delayed modification of at least one signal.
According to the invention this hearing aid is characterized in that said change-over control means comprises controllable attenuation means and controllable time or phase delay means acting on signals derived from the signals from both of the first and second microphone means, respectively, said attenuation and phase delay means being controlled for forming said overall combined signal (Y) using an adjustable attenuation control parameter (omni) and a delay (T), whereby said overall combined signal (Y) is determined by
Y=Xfront*(1xe2x88x92omni*exe2x88x92jxcfx89T)+Xback*(omnixe2x88x92exe2x88x92jxcfx89T),
to change over the hearing aid between said omnidirectional characteristic and any desired form of said directional characteristic as a smooth change-over substantially without affecting phase relationship, time delay and amplitude characteristic of the hearing aid.